[MHml] flying helm (was winch vs tackle)

[S Fishwick]

 -----Original Message-----
From: "Ross Hobson" <r.s.hobson at newcastle.ac.uk
On: Sun, 9 Dec 2007 10:31:41 -0000
<cut>
> I would appreciate someone explaining why some boat
> experience it and other don't - I guess it may be due to
> design/hull shape and its balance with the rudders
>
> Ross


Try this first approximation --

When a cat is upright, e.g. under power in a flat sea, the drag of the
two hulls is equal.

When it heels, the drag of the windward hull decreases as the hull comes
out of the water, and that of the lee hull increases. So the net Centre
of Hull Force (which includes both drag and the sideways force) moves to
leeward.
As an extreme, when the windward hull flies, the CHF aligns
(approximately) with the centreline of the lee hull.
The shift in the line of the drag force causes the boat to tend to bear
away.

HOWEVER ...

As the RIG heels, the Centre of Effort of the rig also moves to leeward,
which causes the boat to tend to luff.

Whether one cancels out the other or not is going to depend on a whole
host of factors - course w.r.t. wind, height of rig CE, hull spacing,
depth of daggerboards vs depth of rudders (primarily on the lifting
hull), shape of the leehull (whether as it immerses more it shifts the
CHF fore-and-aft), change in trim of the lee hull (similar effect) etc
etc.

It's probably predictable, but given that number of variables is not
going to be consistent from one design of boat to another.

Clear as mud??

Simon